The dealer INSTALLED it. They did not set it up for you. They could not have possibly set it up for you because they installed it on a dry empty trailer. You went home and filled the water tank, filled the fridge, filled the drawers, filled the storage compartments, and completely changed the whole dynamic of the rig.
IMHO you should never trust the dealer to have set up the WD hitch properly for you. Remember they already have your money, and they are paying their guys by the hour. The less time a mechanic spends on each task means more money in the owner's pocket.